The present invention relates to wood golf club shafts, i.e., shafts which are adapted to receive a wood club head, and especially to a wood golf club shaft exhibiting a low kick point.
A common characteristic of golf club shafts, especially wood golf club shafts, in the process of being played is a tendency for the shaft to flex or bend backwardly during the downstroke. Consequently, the amount of momentum and directional control imparted to the ball is enhanced. Accordingly, many golf club shafts have been designed to promote such a flexing action.
The point along the shaft about which a maximum flexing occurs has various names, such as "kick point", "flex point" and "whip point" for example. Shafts have been designed with a so-called high kick point located nearer to the butt of the shaft, a low kick point located nearer to the tip, and a middle kick point located at or near the shaft midpoint.
The location of the kick point along the shaft influences shaft playability by affecting the angle of the shaft at the moment of contact with the ball. That is, the lower the location of the kick point, the greater the angle of trajectory of the ball, i.e., the higher the ball travels. Thus, golfers who might otherwise tend to hit the ball too low can benefit from a low kick point shaft. Shafts with high and low kick points are depicted schematically and somewhat exaggeratedly in FIGS. 8 and 9, respectively, to contrast the high and low angles.
A shaft also exhibits a certain "feel" to a golfer which is a function of the overall flexibility of the shaft. Flexibility of the shaft can be determined by measuring a static flex characteristic of the shaft.
One conventional wood club shaft B produced by the present assignee is depicted in FIG. 7. That shaft is formed of metal such as titanium and includes a gradually tapered section 1 which interconnects butt and tip sections 2, 3 of the shaft. The butt section has a standard outer diameter d1 of 0.6 in., and the tip section has a standard outer diameter d2 of 0.335 in. The total shaft length L' is a standard 45 in. The tapered section 1 tapers down from the butt section 2 to the tip section 3 at a rate of 0.011 in./in. (i.e., the outer diameter changes by 0.011 in. per in. of length). The tapered section has a length L1' of 24.5 in., and the section of the shaft wall forming the taper gradually increases toward the tip section from a minimum thickness t1 of 0.018 in. to a maximum thickness t2 of 0.03 in. The tapered section is formed by two successive swaging operations.
In one embodiment of that shaft B the butt section length L2' is 15.5 in. and the tip section length L3' is 5 in. The static flex of the shaft is 5.94 in. The kick point of the shaft is located at a distance L4' of 28.75 in. from the butt end of the shaft. (The manner of measuring the flexure and the location of the kick point will be described hereinafter.)
In a stiffer version of the shaft B of FIG. 7, the butt section length L2' is 16.125 in. and the tip section length L3' is 4.375 in. The kick point of the shaft is located a distance L4' of 28.0 in. from the butt end of the shaft. The static flex of that shaft is 5.6 in. reflecting a greater stiffness than the earlier described embodiment.
It may be desired to change the location of a kick point of a wood club shaft in order to appeal to golfers whose style of play would benefit from such a change. This can be achieved by changing the relative strength of the butt and tip sections 2, 3 of the shaft. For example, in order to lower the kick point of a shaft, the tip section of the shaft would be weakened relative to the butt section, or the butt section would be strengthened relative to the tip section. Among the techniques which have been proposed for U.S. Pat. Nos. 2,250,428 and 4,123,055. The former patent explains how a kick point or whip point can be established by creating a step in the shaft, i.e., by reducing the diameter of the shaft at a suitable location. This reduced diameter portion lies between upper and lower shaft portions of greater diameter. Such a shaft structure is difficult to manufacture and is prone to breakage at the reduced diameter portion which serves to weaken the shaft. Furthermore, the static flex characteristic of the shaft is changed so that the club no longer has the same "feel" to the golfer.
The technique disclosed in afore-mentioned U.S. Pat. No. 4,123,055 involves the provision of a short and very steep taper, e.g., 25.degree. to 45.degree., at a location spaced 5 to 10 in. from the lowermost end of the shaft. Such a steep taper may also tend to unduly weaken the shaft as well as to change the feel of the shaft.
It would be desirable, therefore, to be able to change the location of the kick point of a wood golf club shaft without materially weakening the shaft or changing the static flex characteristic of the shaft, whereby the shaft would exhibit the same feel to a golfer who is accustomed to that shaft.